Nice summation WD
I'm not entirely convinced by those water results... but at the same time with big daily water changes, and with the fact they aren't likely to be adult fish it's not alot of bioload on the tank and if the tap has low nitrates anyways...
I wouldn't be doing anymore daily water changes, I'd give things a chance to settle for 24 hours.
But all these deaths... I can only think it's either bad stock, or that they really suffered during transport.
As for the frogs... I'm not a frog person either but I've read quite a few of the threads. I personally wouldn't have them in such a large tank going by what I've read. They seem to be quite timid, and also need to breath air. In the current situation to breath air they have to make a very long dash passing quite a few fish and they may genuinely just be afraid. Not afraid in a human sense, but their natural instinct to survive may have kicked in, and feeding isn't top of the agenda.
I would be tempted to try them in a 10-20l tank/tub that is filtered and heated. Give them some cover and see if they perk up. Also, I doubt the salt is doing them much good. They are 100% freshwater, I'm only speculation on this but maybe they're being irritated by the salt?
You're definitely feeding them right, there are more things that they can eat. But from what I've seen live bloodworm will be the thing to get them eating if nothing else works.